1. Introduction

Since the bulk of the heating of the earth by greenhouse gas warming
appears to have been taken up by the World Ocean (Levitus et al. 2005), estimates
of interannual World Ocean heat storage changes (Willis
et al. 2004; Levitus
et al. 2005) are very important in evaluating climate model performance (Barnett
et al. 2005), understanding the energy imbalance of the earth associated with
global warming, and estimating the earth’s climate sensitivity to changes
in greenhouse gas concentrations and other climate forcing (Hansen
et al. 2005).

Most estimates of changes in World Ocean heat storage have been limited to
the upper 750 m (Willis
et al. 2004), or at most the upper 3000 m (Levitus
et al. 2005), because historical temperature data become very sparse below
the 750-m depth limit of most expendable bathythermographs (XBTs). The growing
array of Argo floats (more information is available online at http://www.argo.net)
promises to enhance routine ocean measurements of the ice-free World Ocean
compared to the past XBT-based system by achieving even global coverage, adding
measurements of ocean salinity to those of temperature, sampling to 2000 m,
and sampling throughout the annual cycle. All of these things provide a great
improvement for climate science, but data for estimates of deep (>2000 m)
ocean heat storage changes will still be very sparse.

As might initially be expected for the case where heat is simply mixed down
from the surface of a stratified fluid like the ocean, heat content changes
do appear to be surface-intensified (Willis
et al. 2004; Levitus
et al. 2005).
For example, simple linear fits to World Ocean heat content variations for
0–300-m and 0–700-m analyses of Levitus
et al. (2005) between 1955 and 1998
have, respectively, slopes that are 35% and 59% of the slope for the 0–3000-m
analysis (not shown), even though those layers span only 10% and 23% of the
depth of the 0–3000-m layer, respectively.

However, the ocean is not ventilated solely by mixing from a shallow surface
mixed layer into the thermocline. At high latitudes in locations such as the
Labrador Sea (Lazier
et al. 2002) and the Greenland Sea (Karstensen
et al. 2005), very dense waters occasionally form where cooling in the open ocean
is sufficiently strong to overcome the weak local stratification and create
a surface mixed layer that extends deep into the water column, thus locally
exposing the abyss to surface forcing. In addition, very dense waters are formed
on ocean shelves around Antarctica, which then cascade down into the abyss
(Orsi
et al. 1999). Combinations of these North Atlantic Deep Waters and Antarctic
Bottom Waters ventilate the cold deep abyss, mixing with waters above them
(Mantyla
and Reid 1983). As a result, while middepth waters in the Pacific
and Indian Oceans are some of the “oldest” waters in the world in terms of
the time since they have last been exposed to the surface (or ventilated),
the bottom waters are significantly newer (England
1995).

Abyssal cooling of about 0.02°C has been reported in the southwest Pacific
Ocean in 1990/91 relative to 1968/69 (Johnson
and Orsi 1997). It should be
borne in mind that the deep stations they analyzed were widely spaced in the
horizontal, not all these deep stations were occupied all the way to the
bottom, the 1968/69 stations had about 500-m vertical spacing between samples
in the abyss, and 0.01°C is about the best instrumental accuracy expected
for the reversing thermometers (Emery
and Thomson 1998) that were used in 1968/69.
In contrast, more recent analyses of modern closely sampled high-quality repeat
hydrographic section data taken over the last decade or so reveal an abyssal
warming of 0.005°–0.01°C at decadal intervals in the very coldest,
nearly vertically homogenous abyssal waters of the main deep basins of
the Pacific Ocean that are ventilated from the south (Fukasawa
et al. 2004;
Kawano
et al. 2006b).

Here deep ocean temperature differences are presented from analyses
of modern high-accuracy closely spaced hydrographic section data taken in the
Pacific Ocean from the Antarctic Circumpolar Current to the Alaskan Stream
and occupied at least twice during the past few decades (Fig.
1). Some of these
differences are new and some are previously reported but are reanalyzed here
in a slightly different way. The results presented here add to and confirm
pioneering findings of abyssal Pacific Ocean warming in recent decades (Fukasawa
et al. 2004; Kawano
et al. 2006b). The warming is estimated to be statistically
significant in many locations. Furthermore, the potential contribution of this
abyssal warming to the global heat budget is discussed.